Typically, recording media used in printing processes degrade when exposed to chemical or physical processes involving the environment or involving the printing process itself. A recording medium is a composition capable of providing an indicator, a surface modification, or an aesthetic attribute on a substrate. Recording media can include, for example, inks used in printing on paper and textiles, surface modifiers that provide gloss or texture, colorless compositions that change to color when irradiated with UV or visible radiation, various coatings for surfaces, and the like.
Photodegradation occurs when the recording medium fades when exposed to electromagnetic radiation such as sunlight or artificial light and the like. These degradation mechanisms include photooxidation or -reduction of the recording medium depending upon the environmental conditions experienced. Product analysis of stable photoproducts and intermediates in various recording media has revealed several important modes of photodegradation. These include electron ejection from the recording medium, reaction with ground-state or excited singlet state oxygen, bond cleavage to form various products, reduction to form the colorless leuco dyes, and electron or hydrogen atom abstraction to form radical intermediates.
Various environmental factors such as temperature, humidity, gaseous reactants including O2, O3, SO2, and NO2, and water soluble nonvolatile photodegradation products have been shown to influence fading of colorants. The factors that effect fading appear to exhibit a certain amount of interdependence. It is due to this complex behavior that observations for the fading of a particular colorant on a particular substrate cannot be applied to colorants and substrates in general.
The ability of a light source to cause photodegradation in a recording medium is also dependent upon the spectral distribution of the light source, that is the proportion of radiation of wavelengths most effective in causing a change in the recording medium and the quantum yield of degradation as a function of wavelength. On the basis of photochemical principles, it might be expected that light of higher energy (short wavelengths) would be more effective at causing fading than light of lower energy (long wavelengths). Studies have revealed that this is not always the case. Over 100 colorants of different classes were studied and found that generally the most unstable were faded more efficiently by visible light while those of higher lightfastness were degraded mainly by ultraviolet light (McLaren, K., J. Soc. Dyers Colour, 1956, 72, 86).
In addition, the influence of a substrate on recording medium stability can be extremely important. Due to the complex behavior of recording media, the mechanisms causing fading of a particular recording medium on a particular substrate cannot be applied to recording media and substrates in general. Fading may be retarded or promoted by a chemical group within the substrate. Such a group can be a ground-state species or an excited-state species. The porosity of the substrate is also an important factor in recording medium stability. For example, a high porosity can promote fading by facilitating penetration of moisture and gaseous reactants into the substrate. A substrate may also act as a protective agent by screening the recording medium from light of wavelengths capable of causing degradation.
The purity of the substrate is also an important consideration whenever the photochemistry of dyed technical polymers is considered. For example, technical-grade cotton, viscose rayon, polyethylene, polypropylene, and polyisoprene are known to contain carbonyl group impurities. These impurities absorb light of wavelengths greater than 300 nm, which are present in sunlight, and so, excitation of these impurities may lead to reactive species capable of causing fading (van Beek, H. C. A., Col. Res. Appl., 1983, 8(3), 176).
Under conditions of constant temperature it has been observed that an increase in the relative humidity of the atmosphere increases fading of a colorant for a variety of colorant-substrate systems (e.g., McLaren, K., J. Soc. Dyers Colour, 1956, 72, 527). For example, as the relative humidity of the atmosphere increases, a fiber may swell because the moisture content of the fiber increases. This aids diffusion of gaseous reactants through the substrate structure.
In addition to fading, recording media tend to bleed when applied to certain substrates, especially textiles. Accordingly, a recording medium capable of demonstrating enhanced stability and light fastness when applied to any type of substrate is desired.
There is also a need for a recording medium that not only provides increased stability and lightfastness, but also one that is capable of being printed on substrates without special treatment or other limitations. In addition, a superior textile recording medium with substrate independent durability performance is needed.
What is also desired is a recording medium that not only provides increased stability and lightfastness, but also one in which color intensity and hue are capable of being finely manipulated.
There also exists a need for methods and compositions which are capable of stabilizing a wide variety of recording media from the effects of both sunlight and artificial light. In addition, methods and compositions are needed that can stabilize a recording medium from the deleterious effects of humidity and oxygen and other gaseous reactants such as O3, SO2, and NO2.